For example, Japanese Unexamined Patent Publication No. 2003-244792 (Patent Document 1) discloses a transparent piezoelectric film speaker is mounted on a display screen of a cellular phone as being bent in order to output sound from a wide range to enhance a hearing performance from the speaker.
However, mounting the transparent speaker on the display screen as being bent not only deteriorates the visual quality of the device such as a cellular phone, but also sometimes deteriorates visibility of the display screen, since things around it are reflected on the transparent speaker.
Moreover, in the transparent speaker disclosed in Patent Document 1, the side face of the vibration plate is greatly open, so that dusts or other smudges are easy to enter, and once they enter, removing them is a troublesome operation. Further, a protection plate for protecting the display screen and the transparent speaker cannot be shared. If they are shared, waterproof property of the display device is remarkably reduced, since the side face of the transparent speaker is greatly open.
Patent Document 1 discloses PVDF (polyvinylidene fluoride) as an example of a material of a piezoelectric film (piezoelectric sheet) provided to the piezoelectric speaker. FIG. 55 graphically illustrates a result of a displacement calculated by a finite element method, the displacement being generated when electrodes are formed on both surfaces of a PVDF sheet to which a poling is performed in the thickness direction, and two sheets are bonded to form a bimorph 100. In the speaker disclosed in Patent Document 1, the original state before the deformation is bent, but in FIG. 55, the original state before the deformation is defined as a plane, and how it is deformed when voltage is applied thereto is illustrated. It is to be noted that the displacement is illustrated more exaggeratingly than the actual one. When a reverse voltage is applied, it is deformed such that the central portion is sagged downward, contrary to the deformation illustrated in FIG. 55. When an alternating-current voltage is applied, the deformations described above are repeated, which produces vibration to make a sound.
FIG. 55 illustrates the displacement when opposing two sides of the bimorph 100 are fixed. In other words, the bimorph 100 vibrates as illustrated in FIG. 55 when the opposing two sides are fixed, but it hardly vibrates when four sides are fixed, for example.
Therefore, when the bimorph 100 is adopted to a speaker, it is required that the bimorph 100 can be vibrated, and therefore, restriction is imposed on the design for fixing the bimorph 100. When the application of the bimorph 100 to the speaker is considered, it is demanded that the bimorph 100 can be vibrated even if it is fixed on four sides, i.e., on the whole circumference.
Patent Document 1: Japanese Unexamined Patent Publication No. 2003-244792